Photon upconversion is a promising wavelength-shifting technology for photon management. This multi-photon process has potential applications in biological imaging, photocatalysis and photovoltaics.
Multi-excitonic processes can be harnessed to reorganize the energy contained in photons in order to improve the performance of photovoltaic devices or photocatalysts. Reshaping the solar spectrum to match the optical properties of common semiconductors will allow the efficient use of all incident light. While many efforts e.g. hot carrier devices, intermediate band or multi-exciton generation solar cells, offer a route to manipulating incoming photons, the conversion of low energy near-infrared (NW) photons to higher energy photons is particularly appealing, especially when considering NIR radiation comprises 53% of the solar spectrum.
The upconversion of NIR photons at the solar flux has not been demonstrated. If this formidable challenge is met, sub-bandgap photons that are currently not absorbed by common semiconductors can be utilized. Photon upconversion is predicted to increase the power conversion efficiency of a single p-n junction silicon solar cell from 28% to 43%, beyond the Shockley-Queisser limit. Currently, the upconversion of incident photons at power densities commensurate with the solar flux has only been demonstrated for the conversion of green to violet light, via a triplet-triplet annihilation (TTA) based mechanism. This is because other upconverting platforms, like the lanthanides or the chromophores for multi-photon absorption (used in bioimaging) require high excitation densities for appreciable efficiency. TTA-based photon upconversion can be efficient when molecular or nanocrystal (NC) light absorbers are used to sensitize triplet states on molecules. Two triplets can encounter each other and undergo TTA to emit a high-energy photon. Internal upconversion quantum yields (QYs) as high as 35% and 14% have been reported for the upconversion of green to violet light with palladium porphyrins and CdSe NCs as sensitizers respectively. However, in terms of harvesting NIR photons, molecular sensitizers that absorb strongly in the NIR generally have low fluorescence QYs due to strong internal conversion, as predicted by the energy gap law. In contrast, the size, shape and material dependent optical properties of NCs make them ideal as light absorbers for photon upconversion.
From the foregoing, it can be seen that there is a need in the art to prepare nanocrystal transmitter ligands that can provide general, reliable, and efficient upconversion of low energy near-infrared incident photons to higher energy photons. The present disclosure provides this and other advantages as well.